Background/purposeCrack lines (CLs) in cracked teeth (CTs) are challenging to detect. Since restoring CT by occlusal veneer showed favorable clinical outcomes, we found that CLs could be identified more clearly after tooth preparation during the treatment. Herein, we reevaluated and recorded the location of CLs and analyzed their associations with the prognosis.Materials and methodsEighty CTs with normal or reversible pulpitis were assessed and restored by occlusal veneer. Before and after preparation, CLs were identified and divided into type I: CLs without mesial-distal or buccal-lingual direction across the marginal ridge and type II: CLs with mesial-distal or/and buccal-lingual direction across the marginal ridge. Patients were followed up for 1 week, 1, 2, 3, 6, and 12 months after treatment to examine the relief of thermal/biting sensitivity and pulp complications.ResultsThe groups underwent significant changes before and after tooth preparation. The classification before tooth preparation was not related to probing depth and the prognosis of CT; however, after tooth preparation, CTs in type II were correlated with probing depth >6 mm (r = 0.271, P = 0.015), while teeth in type I experienced a short time relief (≤1 month) in thermal sensitivity compared to those in type II (P = 0.013). There was also a significant difference between type I and type II in the survival rate of pulp vitality (P = 0.029).ConclusionCLs could be identified accurately after tooth preparation, impacting the relief time of symptoms and pulp survival rate in CTs.